The first federally approved clinical trial of retroviral vector based gene therapy for human genetic disease has begun. There are several concerns that must be addressed when applying retroviral-mediated gene transfer to human somatic cell genetic diseases. One concern is that the vector, or components of it, may have oncogenic effects. We are investigating whether the use of retroviral vectors for gene therapy may have adverse consequences by examining the effects of such vectors on transformation of cells in vitro. The initial phase of the project involved infecting C3H10T 1/2 cells with the retroviral vector backbone PE501/G1Na (LTR-neo-LTR), ecotropically packaged and supplied by Dr. Anderson (NIH). This vector was found to efficiently infect C3H10T 1/2 cells and to confer resistance to the antibiotic G418, to transfected cells. Transformation experiments were performed under three different regimens: a single exposure to the vector at varying titer, repeated exposures, and exposure followed by replating of the cells. Southern blots show that the vector integrated into the DNA of the infected cells and that the number of integrated vector copies increased with increasing titer of the vector. Each experiment measured three effects: cytotoxicity, G418 resistance, and transformation. 3- methylcholanthrene (3-MC), a known carcinogen which is transforming but relatively nontoxic, served as a positive control. In no case did exposure to the vector, PE501/G1Na, cause any decrease in survival or increase in transformation of C3H10T 1/2 cells compared to untreated (negative) controls.